Steam reforming is widely practised and is used to produce hydrogen streams and synthesis gas for a number of processes such as ammonia, methanol as well as the Fischer-Tropsch process. In a steam reforming process, a desulphurised hydrocarbon feedstock, e.g. methane, natural gas or naphtha, is mixed with steam and passed at elevated temperature and pressure over a suitable catalyst, generally a transition metal, especially nickel, on a suitable support. Methane reacts with steam to produce hydrogen and carbon oxides. Any hydrocarbons containing two or more carbon atoms that are present are converted to carbon monoxide and hydrogen, and in addition, the reversible methane/steam reforming and shift reactions occur. The extent to which these reversible reactions proceed depends upon the reaction conditions, e.g. temperature and pressure, the feed composition and the activity of the reforming catalyst. The methane/steam reforming reaction is highly endothermic and so the conversion of methane to carbon oxides is favoured by high temperatures. For this reason, steam reforming is usually effected at outlet temperatures above about 600° C., typically in the range 650° C. to 950° C., by passing the feedstock/steam mixture over a primary steam reforming catalyst disposed in externally heated tubes. The composition of the product gas depends on, inter alia, the proportions of the feedstock components, the pressure and temperature. The product normally contains methane, hydrogen, carbon oxides, steam and any gas, such as nitrogen, that is present in the feed and which is inert under the conditions employed. For applications such as Fischer-Tropsch synthesis, it is desired that the molar ratio of hydrogen to carbon monoxide is about 2 and the amount of carbon dioxide present is small.
In order to obtain a synthesis gas more suited to Fischer-Tropsch synthesis, the primary reformed gas may be subjected to secondary reforming by partially combusting the primary reformed gas using a suitable oxidant, e.g. air or oxygen. This increases the temperature of the reformed gas, which is then passed adiabatically through a bed of a secondary reforming catalyst, again usually nickel on a suitable support, to bring the gas composition towards equilibrium. Secondary reforming serves three purposes: the increased temperature resulting from the partial combustion and subsequent adiabatic reforming results in a greater amount of reforming so that the secondary reformed gas contains a decreased proportion of residual methane. Secondly the increased temperature favours the reverse shift reaction so that the carbon monoxide to carbon dioxide ratio is increased. Thirdly the partial combustion effectively consumes some of the hydrogen present in the reformed gas, thus decreasing the hydrogen to carbon oxides ratio. In combination, these factors render the secondary reformed gas formed from natural gas as a feedstock more suited for use as synthesis gas for applications such as Fischer-Tropsch synthesis than if the secondary reforming step was omitted. Also more high grade heat can be recovered from the secondary reformed gas: in particular, the recovered heat can be used to heat the catalyst-containing tubes of the primary reformer. Thus the primary reforming may be effected in a heat exchange reformer in which the catalyst-containing reformer tubes are heated by the secondary reformed gas. Examples of such reformers and processes utilising the same are disclosed in for example U.S. Pat. No. 4,690,690 and U.S. Pat. No. 4,695,442.
WO 00/09441 describes a process wherein a feedstock/steam mixture is subjected to primary reforming over a catalyst disposed in heated tubes in a heat exchange reformer, the resultant primary reformed gas is subjected to secondary reforming by partially combusting the primary reformed gas with an oxygen-containing gas, the resultant partially combusted gas then being brought towards equilibrium over a secondary reforming catalyst, and the resultant secondary reformed gas used to heat the tubes of the heat exchange reformer. In the process, no hydrocarbon feedstock by-passes the primary reforming stage. Carbon dioxide is separated from the secondary reformed gas before or after its use for the synthesis of carbon containing compounds, and is recycled to the primary reformer feed. In one embodiment described in WO 00/09441, the recycled carbon dioxide is part of the tail gas from a Fischer-Tropsch synthesis process, and is added to the natural gas feedstock prior to desulphurisation of the latter.
Fischer-Tropsch tail gas is liable to contain a significant amount of carbon monoxide. If this is added to the feedstock prior to primary reforming in a heat exchange reformer, the carbon monoxide undergoes the exothermic methanation reaction resulting in a faster increase in temperature of the gas undergoing reforming than if the tail gas had not been added. The temperature difference between the gas undergoing reforming and the heating medium is thus decreased and so more heat transfer area, e.g. more and/or longer heat exchange tubes, is required for a given reforming duty.
In our co-pending application PCT/GB 02/03311 we have demonstrated that this problem may be overcome by addition of the Fischer-Tropsch tail gas to the primary reformed gas before partial combustion thereof, i.e. addition of tail gas to the primary reformed gas between the steps of primary and secondary reforming. Such addition, where carbon dioxide is present in the tail gas or is added from another source, further has the effect of allowing lower steam ratios to be used in the primary reformer. [By the term “steam ratio” we mean the ratio of the number of moles of steam to the number of gram atoms of hydrocarbon carbon in the feed: thus a methane/steam mixture comprising 2 moles of steam per mole of methane has a steam ratio of 2.] This has advantages in respect of providing lower operating costs, for example in steam generation.
Use of lower steam ratios, for example steam ratios below 1.00, can, however, lead to carbon formation on the exposed surfaces of the catalyst. Such carbon formation has the undesirable effect of increasing the pressure drop through the catalyst. It can also result in a loss of catalyst activity. Thus there is a desire to use lower steam ratios than those previously achieved without the risk of increasing carbon deposition.